Filter or function in TWIG?

Question

When should some functionality be created as a filter instead of a function and vice versa?

user | function

vs

function(user)

Someone pointed out the documentation, however it does not explain when to use which. It does say this:

Functions support the same features as filters, except for the pre_escape and preserves_safety options.

What is the point of twig support function if they behave just like a filter?

Answer 1

A filter is a way to transform the displayed data. For example if you want to display the content of a variable L1K3 Th1s, you'll have to write a filter (example : {{ username|l33t }})

A function is used when you need to compute things to render the result. For example, the {{ dump(username) }} function which will call internally the var_dump php function.

Usually you write a function when you need to do heavier things than just simply transform the display of a content in a simple way.